The instant invention is in the field of porous metal structures. More specifically, the instant invention relates to porous metal structures having elongated pores and/or porous metal structures having relatively high density and methods for producing such structures.
A number of metal foam structures have been reported; see, for example the text book by Ashby et al., Metal Foams, A Design Guide, 2000, ISBN 0-7680-0555-8. Metal foams are characterized as having open cell topology or closed cell topology. The metal of either structure is a continuous phase. The cells of the closed cell structure are a discontinuous phase entrapped in the continuous metal phase. The cells of the open cell structure are interconnected to form a continuous tortuous phase intermingled with the continuous metal phase.
Open cell porous metal structures to be used, for example, as substrates for battery electrodes can be made by depositing a metal (such as nickel) onto the surfaces of an open cell polymer foam preform (usually open cell polyurethane foam) followed by vaporization of the polymer, see, for example, U.S. Pat. Nos. 4,076,888; 4,957,543; and 5,738,907. The '907 patent pointed out that the pores of such open cell porous metal structures tend to be somewhat oval in shape but can be made to be more isotropic (not unlike the combination of a circle and a polygon) by appropriate stretching of the open cell polymer foam preform in a direction transverse to the direction of production of the foam.
The cell size of open cell porous metal structures ranges from about 0.1 millimeter to about 2 millimeters, Ashby, supra. The relative density (relative to solid metal) of open cell porous metal structures ranges from as low as 0.02 to about 0.25, Ashby, supra. The relative density of sintered metal structures (such as sintered bronze bearings) ranges from about 0.6 to about 0.88.
In 1977 Frank E. Towsley was granted a patent on a unique open cell porous metal structure made, for example, by electrodepositing a metal in the interstitial spaces of a compacted bed of polystyrene particles followed by dissolution of the polystyrene, see U.S. Pat. No. 4,053,371, herein fully incorporated by reference.
Towsley used such a porous cellular metal, for example, in an improved electrolytic cell; see U.S. Pat. No. 4,121,992, herein fully incorporated by reference. Towsley suggested a number of other applications such as a filtration membrane, an electrode assembly for batteries, lightweight structural members, impact energy absorbers, and abrasive grinding combinations.
The cell diameter of Towsley's open cell porous metal structure can be relatively small (for example, 0.0001 millimeter) to relatively large (for example, 1 millimeter or more) depending of the diameter of the polystyrene particles used. The relative density of Towsley's open cell porous metal structure ranged from about 0.01 to about 0.5.
The aspect ratio of an anisotropic pore of an open cell porous metal structure is the ratio of the larger diameter of the pore divided by the smaller diameter of the pore. Due primarily to surface tension forces, the pores of open cell porous metal structures tend to be generally spherical in shape. However, it would be an advance in the art of open cell porous metal structures if the aspect ratio of the pores were greater than 1.5. For example, pores having an aspect ratio of greater than 1.5 (at least at the surface of the filter) are preferred for filtering particulate materials from fluids so that the filtered particulate does not blind or block the pore. In addition, it would be an advance in the art if the relative density (relative to solid metal) of open cell porous metal structures could be increased over prior art structures.